The analysis of the volatiles content of materials is one of the most common types of testing carried out on a wide variety of materials. In its most fundamental sense, the measurement of the volatile (very often the moisture) content of a material is usually carried out by taking a representative sample of the material, weighing it, drying it, and then reweighing it after the drying process. The difference between the starting weight and the ending weight, when divided by the starting weight and expressed as a percentage, is the percentage of volatile content. The procedure is based on differential weighing and is somewhat formally referred to as gravimetric moisture determination.
As a preliminary matter, it is well-understood in this art that the results of such differential weight analysis can be calculated and expressed either as the percentage of volatiles that leave a sample, or as the percentage of solids that remain. The manipulative steps, however, are the same. Thus, devices of the type described herein are often described as "moisture/solids" analyzers.
Furthermore, although the term "moisture" is used most frequently herein, it will be understood that the apparatus and methods disclosed and claimed herein apply to any volatile species that can be driven from a heated sample in a differential weighing technique.
For many years, moisture analysis of this type has been carried out by heating the respective samples in conventional conduction or convection-type ovens. The process is generally time consuming for several reasons. First, in order to make sure that all of the moisture has been driven from a sample, the heating and re-weighing process must be carried out several times until the difference in dry weights are either eliminated, or so small as to be within measurement accuracy limits. Because conventional drying is relatively time consuming, the need to take several repeated measurements on the same sample is similarly time consuming.
As another factor, the analytical balances typically used to weigh the samples are usually pan-type balances; i.e., they include a flat surface upon which the sample (and sometimes its container) are placed and which is in turn attached to a sensitive balancing mechanism. Under these circumstances, when a warm object such as a heated sample is placed on a balance pan, the warm object tends to set up a flow of convection air currents in the immediate vicinity. In turn, these upwardly flowing currents tend to lift the balance pan and produce an inaccurate reading. Generally, the more sensitive the balance, the more likely, or the greater amount proportionally by which, the reading will be in error. Thus, in addition to the time required to conventionally heat a sample to dryness on a repeated basis, there also exists the need to allow the sample to cool sufficiently to avoid creating convection currents in the balance. Thus, convection and conduction methods of moisture analysis tend to be relatively time consuming.
More recently, microwave energy has been used to help speed up the drying process. In these techniques, microwaves are used to drive the moisture from the sample rather than conventional convection or conduction heating. Microwaves offer several advantages in this respect, the most direct being the fact that they heat water or other volatiles directly, rather than by conduction through the material itself. Stated differently, microwaves immediately interact with the moisture in a sample and tend to volatize it quickly. Furthermore, because microwaves affect only certain types of substances within a sample (generally polar substances), they tend to heat the overall sample less than do conventional convection and conduction techniques. As a result, microwave heating can hasten moisture analysis by several orders of magnitude. By way of example, different manufacturers of microwave drying devices point out that processes that can take as many as 16 hours for moisture analysis (e.g., ground beef) can be done in a conventional microwave device in about 5 minutes. Additionally, even those materials which can be dried relatively quickly in conventional ovens, can still be greatly accelerated in a microwave device. For example, according to published information, tomato paste (which has a moisture level of about 77.55%) can be dried in a conventional oven in about 1.5 hours. In contrast, it can be dried in a conventional microwave device in about 5 minutes. Other materials, such as cheese, can be analyzed in a conventional microwave drying device in as little as 3.5 minutes.
Accordingly, the microwave moisture analyzer has become a widely accepted piece of apparatus in many chemical laboratories. An exemplary version is the Lab Wave-9000 microwave moisture/solids analyzer from CEM Corporation, the assignee of the present invention, which combines an analytical balance with its microwave drying capability. Devices such as the Lab Wave-9000 are also typically operated in conjunction with microprocessors and related electronic circuits. The processor's operation and logic (software) enable the moisture content to be calculated even more quickly using generally well understood relationships between weight loss over a given period of time and the ultimate moisture content of a given sample.
Nevertheless, the wide acceptance of microwave drying devices such as the Lab Wave-9000 has driven further innovation and a desire to carry out drying processes even more quickly and efficiently. Accordingly, newer types of microwave devices are appearing in the marketplace which attempt to focus microwave energy more carefully and efficiently in an attempt to dry materials more quickly, more accurately, and using less power.
Some newer versions of microwave drying devices tend to follow the disclosures of U.S. Pat. No. 5,632,921 to Per O. Risman. The Risman patent describes a generally cylindrical microwave cavity device in which the modes of microwave energy are carefully controlled to give a heating effect. Based on public disclosures, and without attempting to speak for the respective manufacturers, the "Moistwave" system from Prolabo (France) and the "M2" microwave moisture/solids analyzer from Denver Instruments (Arvada, Colo., USA) appear to follow the Risman patent in their use of specifically cylindrical cavities. According to advertising materials for both of these devices, they are able to reduce even the ordinary microwave drying time by a significant fraction, for example taking drying steps that require 3, 4, or 5 minutes in a conventional microwave device, and carrying them out in about 1 or 2 minutes or less. As set forth in the Risman '921 patent, however, these devices operate on a very carefully selected combination of limited transverse magnetic modes.
The Denver Instruments device also appears to follow some or all of the disclosures of published International Application WO 99/40409 corresponding to International Application PCT/US01866.
Accordingly, a need exists for further improvement in the area of high-efficiency microwave moisture analyzers.